1. Field of the Invention
The present invention relates to an electrophotographic copying apparatus and process and, more particularly, to a copying process which employs a combined developing-cleaning magnetic brush unit and which produces a reversal image of the object copied.
2. Description of the Prior Art
Copying machines employing the electrophotographic process utilize a photosensitive element, usually in the form of a drum or belt, which is imagewise exposed to produce a latent electrostatic image upon the photosensitive element. Toner particles are transferred to the electrostatic image by means of a developer unit and are again transferred to a final receptor, usually a sheet of plain bond paper, to produce a permanent copy of the object. Some toner particles usually remain on the photosensitive element and must be removed by a cleaning unit after the toned image has been transferred to the copy paper. U.S. Pat. No. 3,647,293 discloses that the developer unit and cleaning unit may be combined to reduce the number of elements associated with the copying machine and thereby its size. In that system, a magnetic brush element performs the developing operation during the first revolution of a photosensitive drum. On the second revolution of the drum, a cleaning operation is performed by the magnetic brush element. The combined developing-cleaning magnetic brush element is effective to reduce the size of the electrophotographic copying machine. However, the copying speed is inevitably reduced because the photosensitive drum must rotate twice to complete a one-sheet copying operation.
U.S. Pat. No. 4,465,360 provides a photographic drum which has a circumference which is sufficiently long to incorporate two image areas. By alternating development and cleaning of the two image areas, the rate of copy production is improved over that of U.S. Pat. No. 3,647,293. However, the copying cycle is so arranged that the photosensitive drum must be rotated twice to complete a one-sheet copying operation and three times for every two copies in a multiple copying operation. This copying cycle is effective to increase the speed at which copies are produced, but does not greatly reduce the total number of revolutions of the photosensitive member necessary to produce a given number of copies.
If the photosensitive element is a belt, its life is related to the number of revolutions. So although the cycle of U.S. Pat. No. 4,465,360 is effective to increase the speed at which it may produce copies, the cycle would not greatly extend the useful life of a photosensitive belt, if used.